1. Field of the Invention
The present invention relates to a fiber retaining structure for a photoelectric sensor which is provided on a casing in which a light-emitting element and a light-receiving element are incorporated to retain optical fibers respectively connected to the light-emitting element and the light-receiving element.
2. Description of the Related Art
FIG. 8 illustrates an example of a photoelectric sensor. FIG. 9 is a cross-sectional view of a conventional fiber retaining structure provided on the photoelectric sensor.
As shown in FIGS. 8 and 9, a photoelectric sensor generally includes a body 1, a pair of optical fibers 2 and 3, and a detection head 4 connected to the body 1 via the optical fibers 2 and 3. The body 1 has a casing 5 in which a light-emitting element 6 and a light-receiving element 7 are incorporated, as shown in FIG. 9. The optical fibers 2 and 3 are connected to the light-emitting element 6 and the light-receiving element 7, respectively. In the photoelectric sensor arranged in the manner described above, when the light-emitting element 6 emits light, the emitted light is introduced to the detection head 4 via the optical fiber 2 and is then projected therefrom. If an object 8 exists at a position which opposes the detection head 4, the projected light is reflected by the object 8. The detection head 4 receives the reflected light, and sends it to the light-receiving element 7 via the optical fiber 3 to detect it.
A conventional fiber retaining structure for retaining the optical fibers 2 and 3 includes O-rings 9 into which the optical fibers 2 and 3 are inserted, holding members 10, 11, 12, a connection casing 13 in which the holding members 10, 11, 12 are incorporated, and a tightening screw 14 threaded into the connection casing 13 to press against the holding member 10.
The O-rings 9 are sealing materials which seal the gap around each of the optical fibers 2 and 3 and thereby prevent entry of the water contents into the casing 1. Therefore, the inner diameter of each of the O-rings 9 is smaller than the outer diameter of each of the optical fibers 2 and 3.
In such a conventional fiber retaining structure, the optical fiber 2 is connected to, for example, the light-emitting element 6 in the manner described below. First, the operator holds the optical fiber 2 by his or her hand, and then inserts it into an insertion hole 15 in the connection casing 13.
Consequently, the optical fiber 2 passes through a gap between the holding members 11 and 12 and then through the O-ring 9 while expanding it, and the distal end thereof abuts against the light-emitting element 6. At that time, the operator has a feeling and a confirmation that the distal end of the optical fiber 2 has made contact with the light-emitting element 6 and thus stops pushing in the optical fiber 2. The optical fiber 3 is similarly connected to the light-receiving element 7.
Next, the operator screws the tightening screw 14 with a tool, such as a screw driver. As the distal end of the tightening screw 14 presses against the holding member 10, the optical fiber 3 is held between the holding members 10 and 11 while the optical fiber 2 is held between the holding members 11 and 12, whereby the optical fibers 2 and 3 are retained in a connecting state to the light-emitting element 6 and the light-receiving element 7, respectively. The gap around each of the optical fibers 2 and 3 is sealed by the O-ring 9 so as to prevent entry of the water contents into the casing 1.
In the above-described conventional fiber retaining structure for the photoelectric sensor, since the inner diameter of each of the O-rings 9 is smaller than the outer diameter of each of the optical fibers 2 and 3, when the optical fiber 2, for example, is pushed into the O-ring 9, it expands the O-ring 9 and generates a friction: it is not easy to insert the optical fiber 2 into the O-ring 9.
It is therefore difficult for the operator to have a feeling and hence a confirmation that the distal end of the optical fiber 2 has abutted against the light-emitting element 6 when the optical fiber 2 is pushed in. When the distal end of the optical fiber 2 is not reliably abutted against the light-emitting element 6, the gap between the distal end of the optical fiber 2 and the light-emitting element 6 easily varies. Generation of the gap between the end portion of the optical fiber 2 and the light-emitting element 6 reduces the transmission efficiency of the light from the light-emitting element 6 to the optical fiber 2. A similar problem occurs when the other optical fiber 3 is inserted. Furthermore, when an unskilled operator screws the tightening screw 14 after the distal ends of the optical fibers 2 and 3 have made contact with the light-emitting element 6 and the light-receiving element 7, there is the possibility that the operator may apply an extra force: it is difficult for the operator to apply a proper amount of force.
In addition, the optical fiber retaining process is divided into the optical fiber insertion process and the optical fiber fixing process, and hence has a deteriorated working efficiency.
Furthermore, after the photoelectric sensor in which the optical fibers 2 and 3 are held by the holding members 10 through 12 has been used for a long period of time, the holding force for the optical fibers 2 and 3 may weaken due to the permanent set of the holding members 10 through 12.